1. Field of the Invention
The present invention relates generally to electrophotographic image-forming members or photoreceptors comprising hydrogenated amorphous silicon (a-Si:H) formed onto a supporting conductive substrate. More particularly, this invention is directed to an environmentally stabilized a-Si:H photoreceptor and a method of forming the stabilized photoreceptor.
2. Discussion of the Prior Art
Electrophotography is a well-known image transduction art depending on the formation of an electrostatic latent image on a charge-sensitized photoconductor formed onto a suitable substrate. The latent image is typically produced by photo-induced discharge of the photoconductor in response to a light image projected onto the working surface of the photoconductor, and a visual image for transfer to the hard-copy medium is developed from the latent image by contacting it with charge-sensitive toner particles. The toned image is then the basis for a variety of further imaging processes. The versatility of electrophotography has permitted its application in systems for copying, duplicating, printing, plate making and color proofing, among others, and electrophotography is increasingly being applied in computer output devices in which lasers are used to produce the latent image. Commercial potential of such systems is directly affected by the performance and producibility of the photoconductor. Generally, the photoconductor must have good charge acceptance V.sub.0 and a long dark decay .tau..sub.D, typically 10 to 20 seconds at minimum. In addition, fast photo-induced discharge is required, and the spectral response of the photoconductor must be compatible with the source. In critical applications, photoconductor fatigue or residual voltage may be limiting.
Considerable effort has been expended in development of prior-art photoconductors based on inorganic materials such as cadmium sulfide, zinc oxide, or selenium, as well as organic materials such as TNF-PVCz (the reaction product of 2,4,7-trinitro-9-fluorenone and poly-9-vinylcarbazole). Some prior-art photoconductors suffer well-known disadvantages such as low charge acceptance or short dark decays, poor thermal or environmental stability, poor mechanical strength or durability, or the potential for environmental contamination. Further, others lack good adhesion properties or are otherwise incompatible with use of flexible substrates required by large-format applications such as color proofing. In addition, many require formation temperatures too high to permit their use with plastic substrates.
High-quality, large-format electrophotography can be practiced through use of microcrystalline cadmium sulfide deposited onto thin conductive substrates (U.S. Pat. Nos. 4,025,339 and 4,269,919). A metallic member, or a plastic member coated with a metallic or an ohmic layer, may form such conductive substrate. Sputtered to thicknesses of 0.3 to 5 micra onto stainless-steel roll-stock up to one meter wide and about 0.1 mm thick, such anisotropic photoconductors have been adapted to provide flexible photoreceptors for an analog color-proofing application (U.S. Pat. Nos. 4,358,195 and 4,556,309) This application required that a latent image be retained almost two minutes between photoreceptor charging and development. The large-format (approximately 50 cm by 60 cm ) photoreceptors were required to be reusable for thousands of operational cycles. During operation, at 105 seconds after corona charging these thin cadmium sulfide photoconductors demonstrate typical surface potentials V.sub.105 =22 volts, they have linear photo-induced discharge, and they yield substantially zero residual voltage on complete discharge. When used with optimized liquid toner systems, these flexible low-voltage photoreceptors provide high-resolution four-color proofs. However, a potential environmental hazard due to manufacture or disposal of the cadmium sulfide photoconductor remains a concern.
The disadvantages of prior-art photoconductors has prompted investigation of amorphous silicon (a-Si) as the photosensitive material for use in electrophotographic photoreceptors. Amorphous silicon poses no environmental hazard and has good mechanical strength, adhesion, and durability, but demonstrates undesirable characteristics thought to originate in unsatisfied (or dangling) bonds in the silicon matrix. It has been shown that formation of amorphous silicon in presence of hydrogen provides a material (a-Si:H) with fewer dangling bonds and improved characteristics, the greatest improvement occurring for deposition substrate temperatures of approximately 230.degree. C.
An extensive art based on a-Si:H materials has developed in the field of solar-energy conversion, in which thin a-Si:H layers are routinely deposited onto large-area flexible substrates the internal resistance of such photovoltaic devices must be as low as possible (of the order of 100 ohms) for attractive power outputs, but the corresponding volume resistivities (about 10.sup.6 ohm.multidot.cm) result in photoconductive properties ill-suited to electrophotographic applications. Other a-Si:H materials made to have higher resistivities exhibit attractive photoconductive properties, and by appropriate doping, such a-Si:H photoconductors can be made to accept positive charging, negative charging, or charging in either polarity. However, conventional a-Si:H photoreceptors are typically directed toward rapid-imaging systems for office use, the toner systems for which may require surface potentials of 100 volts or greater but the operational cycles for which seldom require dark decays longer than a few seconds. Consequently, the prior-art a-Si:H photoreceptors (e.g., U.S. Pat. No. 4,265,991) have demonstrated several characteristics which limit their usefulness as low-voltage electrophotographic photoreceptors. Included are the following significant disadvantages:
1. The low dark volume-resistivity (about 10.sup.10 ohm.multidot.cm) of such a-Si:H photoconductors, and their resultant fast dark decays, require deposition of a high-voltage a-Si:H layer at least 10 (and usually 20 to 50 micra in thickness to achieve the surface potentials needed by many electrophotographic processes at toning these thick photoconductive layers are both expensive to produce and poorly adapted to use with flexible substrates. As is known in the art, long dark decays require that a photoconductor have both a wide optical bandgap, which indicates a low density of thermally generated charge carriers, and a low drift mobility for such carriers. The optical bandgap of a-Si:H is known to increase with increasing hydrogen content, up to about 10% total hydrogen, and carrier mobilities in a-Si:H are known to decrease with addition of small amounts of neutral dopants such as oxygen or nitrogen. However, prior-art a-Si:H photoconductors based on either bandgap widening by hydrogen enrichment or mobility suppression by doping-induced trapping enhancement demonstrate degraded photoconductive properties and spatial inhomogeneities in the charge acceptance or toning response. In addition, the bulk properties of prior-art a-Si:H photoconductors are adversely affected by interface processes. When prior-art a-Si:H photoconductors are used in bilayer photoreceptors, carrier injection from the conductive substrate or charge transfer from the environment accelerates bulk dark-decay processes, further reducing applicability of such photoreceptors. Such processes have been partially overcome in the prior art by fabrication of multilayered photoreceptors in which thin (a few hundred nm or less) insulating layers are either deposited at the interface between the a-Si:H photoconductor and the conductive substrate, or topcoated over the photoconductor, or both. PA1 2. Charge injection or impurity migration into the adherent surface of the photoconductor has been particularly limiting for a-Si:H photoconductors formed onto many conductive substrates. In the prior art, thin blocking or barrier layers are commonly deposited on the substrate surface prior to formation of the a-Si:H photoconductor; both electrically insulating and less reactive conductive materials have been used. Yet another approach has been to use the a-Si:H photoconductor as a charge-generation layer and couple it with another layer which acts as a charge-transport layer. These multilayered photoreceptors are complicated to process, costly to produce, and still require a-Si:H layers at least 10 (and typically 20 to 50) micra thick to achieve practical surface potentials at toning; in addition they are inflexible and difficult to manufacture in the large formats required for electrophotographic applications such as analog or digital color proofing. PA1 3. Electrophotographic properties of a-Si:H photoconductors degrade on exposure to environmental humidity or to reactive species present during charging. This sensitivity is thought to originate in unsatisfied dangling bonds on the surface of the a-Si:H photoconductor, as well as on surfaces of internal structural inhomogeneities accessible to active species. In the prior art, such sensitivity has been decreased by overcoating the a-Si:H photoconductor with a thin (of the order of 10 to 200 nm thick) electrically insulating topcoat such as silicon nitride, silicon carbide, or silicon dioxide; however, such prior-art topcoatings add cost, typically require a separate deposition step, and may give rise to an undesirable residual voltage unless kept ineffectually thin. PA1 1. Both the density and refractive index of such a-Si:H photoconductors are notably less than that for crystalline silicon, both decreasing as the substrate temperature at deposition is decreased. It is widely accepted that these effects are due to both formation of microvoids in the photoconductor and segregation of electronically detrimental non-monohydrides and contaminants on the surfaces of such voids. PA1 2. Bonding of hydrogen in non-monohydride modes is favored, with probable concentration in the voids and attendant instability in photoconductor properties. PA1 3. Prior-art a-Si:H photoreceptors formed at substrate temperatures of less than about half of silicon's melting temperature are known to exhibit scanning electron microscope (SEM)-resolvable columnar growth structure 10 to 100 nm in diameter; the columns are separated by interstices due to incomplete coalescence of nucleation islands. Columnar interstices originate at the substrate and propagate through the photoconductor thickness. They not only decrease photoconductor density and refractive index, but also act as segregation surfaces, and serve as diffusion channels for reactive species, so degrading photoconductor properties through increased environmental sensitivity. Such interstitial effects can dominate bulk properties of the columns, particularly in thin layers, and are widely thought to account for the poor electrophotographic performance of prior-art photoreceptors incorporating a-Si:H photoconductors. These effects are particularly limiting if the photoreceptor comprises a thin a-Si:H layer deposited onto a conductive substrate.
These disadvantages typify prior-art a-Si:H photoconductors prepared by either silane-based glow discharge or reactive sputtering based on admixed hydrogen in the sputtering atmosphere. Both preparative methods have been used to deposit prior-art a-Si:H photoconductors onto metal substrates, usually at substrate temperatures of 250.degree. C. or greater. Glow-discharge methods have yielded the best prior-art a-Si:H photoconductors, but deposition rates are low and even further limited if dark resistivities of the order of 10.sup.10 ohm.multidot.cm are to be obtained. In addition, damaging reactions between process gases and plastic substrates preclude deposition of a-Si:H photoconductors onto such substrates by glow discharge methods.
Sputtering methods can achieve deposition rates several times greater than are currently available by glow-discharge processes and thus promise greater commercial utility. Unfortunately, the a-Si:H photoreceptors prepared by prior-art radio-frequency (RF) reactive-sputtering practices suffer especially from the above-listed disadvantages, and it is known that a-Si:H photoreceptors sputter-deposited at substrate temperatures below about 250.degree. C. have particularly poor properties and stability. At such substrate temperatures, sputtering conditions favor several mechanisms considered to be deleterious to a-Si:H photoreceptor performance:
To solve the first two above-listed disadvantages of prior-art a-Si:H photoconductors, a novel photoconductor and method of making the photoconductor is found in the above cross-referenced related application which is herein incorporated by reference. In the related application, the first two above-listed disadvantages of prior-art a-Si:H photoreceptors are overcome through use of novel substrate bias conditions and by controlling hydrogen incorporation during formation of the photoconductor in radio-frequency diode reactive sputtering apparatus, whereby an a-Si:H photoconductor is deposited directly onto a conductive substrate without the intermediate charge-injection blocking or barrier layers of prior-art a-Si:H photoreceptors.
Useful electrophotographic properties are attained for photoconductor thicknesses on the order of one micron, at deposition rates of approximately one micron/hour. Deposited to thicknesses of five micra or less, the new photoconductor yields photoreceptors which exhibit a long dark decay .tau..sub.D, good charge acceptance V.sub.0, and good panchromatic photosensitivity. The new a-Si:H photoconductors attain their significantly greater dark decay with neither the doping nor the alloying used to improve dark resistivities of prior-art a-Si:H photoconductors, but if desired may be doped to provide bichargeable characteristics. Moreover, It has been found that an a-Si:H photoconductor having the required spatially-uniform long dark decays .tau..sub.D can be made at substrate temperatures between 80.degree. C. and 200.degree. C., more preferably between 100.degree. C. and 180.degree. C., with little dependence of the photoconductive properties on deposition temperature within this range. These photoconductors do not demonstrate the marked columnar microstructure typical of prior-art a-Si:H photoconductors formed at substrate temperatures below 250.degree. C., but instead demonstrate a homogeneous glass-like appearance when examined by scanning electron microscopy (SEM).
In contrast to prior-art a-Si:H photoconductors, the new photoconductor contains less than 5% and preferably less than 4% total hydrogen, and due to reduced diffusion of hydrogen at the low formation temperatures, the distribution of hydrogen is more uniform throughout the thickness of the photoconductors than in prior-art a-Si:H photoconductors. The a-Si:H layers contain less than 7% and preferably less than 6% implanted argon, are strongly adherent to the deposition substrate, and provide flexible dopable photoreceptors which can be readily produced in square-meter formats.
The photoreceptor can comprise flexible substrates of metal or plastic provided with a conductive layer, to either of which the new a-Si:H photoconductor is strongly adherent. Due to the low temperatures used in the new method, plastic substrates supporting a conductive or ohmic layer are substantially unaffected by the a-Si:H deposition process. When flexible metallic substrates are used, such photoreceptors may be made by a single deposition process. Deposited onto flexible metallic substrates, such photoconductors demonstrate excellent durability and yield high-resolution, high-quality images useful in various electrophotographic applications such as printing, plate making, duplicating, or color proofing, among others. The photoreceptors are usable over many operational cycles.
A specific example of such a photoreceptor is a bias-sputtered a-Si:H photoconductor five micra or less in thickness, formed onto a conductive substrate with or without an insulating topcoat, as will be referred to hereinafter. In ambient relative humidities below about 40%, such new a-Si:H photoreceptors demonstrate spatially uniform electrophotographic properties of broad commercial potential, and the photoreceptors yield high-resolution, high-quality images when used with imaging processes and liquid toning systems compatible with cadmium sulfide photoreceptors.
However, the new a-Si:H photoreceptors have in common with prior-art a-Si:H photoreceptors the above-listed third disadvantage, a spatially nonuniform charge acceptance and toning response when corona charged under conditions of high ambient humidity. Empirical evidence indicates that this environmental sensitivity originates when water vapor and ions generated from the air under the influence of the corona are absorbed and interact with incomplete bonds on the photoconductor surface, to constitute pathways for leakage of the surface charge. As noted in the prior-art, such environmental sensitivity has been decreased by electrically insulating the active surface sites from the ambient environment by physically top coating the a-Si:H with a thin layer (of the order of 10 to 200 nm thickness) of a dielectric material such as silicon nitride, silicon carbide, or silicon dioxide. However, unless these dielectric top coats are kept thin, their thickness allows three-dimensional electric field gradients to be established which degrade the resolution of the charge (and therefore the reproduced) image. In addition, the prior-art topcoatings add cost due to expensive process materials and decreased process yields, require a separate deposition process in capital-intensive equipment, and can also give rise to an undesirable residual voltage unless they are kept thin (of the order of 5 to 15 nm thickness). The effectiveness of such insulating topcoats as barriers to humidity and reactive species, however, is greatly diminished when they are kept acceptably thin.
It is thus desirable to provide an a-Si:H photoreceptor, with or without an insulating topcoat, having improved environmental stability. It is preferred that the active surface bonds on the a-Si:H photoconductor be chemically completed at the molecular level, rather than electrically insulated from the environment as in the prior art, so that the photoconductor surface retains a functional charge despite presence of water vapor and ions generated by corona charging.